This invention relates to hand-held calculators, and in particular it is concerned with a calculator having transparent background for a liquid crystal display and preferably also for number and function input keys, enabling the calculator to be used with an overhead projector to demonstrate operations of the calculator, along with calculated results, on a screen to a classroom or group.
U.S. Pat. No. 4,154,007 disclosed a hand-held calculator having a transparent window within which a liquid crystal display (LCD) is positioned. This enabled light to be projected through the LCD window and onto a screen or wall to reveal the numbers or symbols as dark elements on the screen with the background left white. The calculator disclosed in the patent was particularly useful with an overhead projector in a classroom, whereby numerical results obtained using the calculator resting on the light platform of the projector could be projected for all in the classroom to view.
Since the above patent was issued, there has been a development by Epson of a pressure-sensitive conductor arrangement useful to produce a clear touch key panel, wherein the pressure of a finger on a key member printed on the back of a transparent membrane will effect sufficient change in conductivity in etched channels of a transparent plate below to act as a pressure switch. The slight change in conductivity is sensed by a circuit which effects a desired result. This technology was dedicated to the public in 1985.
This transparent-key technology has been applied to calculators of the type disclosed in the above referenced patent, with number keys and function are transparent except for the display of a symbol of the number or function. Thus, most of the calculator was transparent and could be projected onto a screen or wall with an overhead projector. In this way, not only numerical results but also the entry of numbers and the pressing of function or operation keys could be demonstrated on the screen using the overhead projector. One such calculator has been marketed by the present applicant, under the trademark "The Educator". A model of that calculator has employed a heavy, opaque grid of lines outlining and delimiting the number and function keys.
Such transparent calculators had certain problems. The transparent key technology required that a glass laminate be used, making the calculator quite fragile if dropped. The calculator could be inadvertently pushed off the glass surface of an overhead projector, for example.
Another problem with these calculators has been heat buildup from the lamp in the overhead projector. Liquid crystal displays are quite susceptible to temperature change, and an increase in temperature will often cause changes in the display and misreadings in the displayed numbers of an LCD calculator of the type described.
A further problem has been in relation to the accuracy of the keys to the applied finger pressure. Due to the nature of the circuitry for implementing these transparent keys, when keys are made smaller and closer together there tends to be some crossover of the effect of pressure on a key, particularly when the pressure is not applied precisely centrally. The result is inaccuracy in the entry of the data or functions to be calculated.